Oxygen desaturation occurs during sleep in some patients with COPD. To investigate the effects of these hypoxemic episodes on the pulmonary vasculature, we studied four patients with our routine polysomnographic techniques and simultaneously recorded pulmonary artery pressure. In all four subjects, nocturnal episodes of desaturation were accompanied by elevations in the pulmonary artery pressure. Low flow oxygen abolished the drops in arterial oxygen saturation (but not the breathing abnormalities) and no elevations in the PA pressure were observed. We postulate that in some COPD patients these initially transient events may lead to sustained pulmonary hypertension and cor pulmonale. Nocturnal oxygen therapy may be indicated in more patients than previously suspected and may prevent the development of cor pulmonale.